The present invention relates generally to motor vehicle wheel alignment and, more specifically, to providing a self-calibrating aligner having multiple cameras for aligning multiple wheels on one side of a vehicle.
Machine vision measuring systems are used in many applications. For example, computer-aided, three-dimensional (3D) machine vision alignment apparatuses and related alignment methods are used in aligning wheels of motor vehicles. Examples of such apparatuses and methods are disclosed in U.S. Pat. No. 5,724,743, entitled xe2x80x9cMethod and Apparatus for Determining the Alignment of Motor Vehicle Wheels,xe2x80x9d issued to Jackson, et al. on Mar. 10, 1998 and in U.S. Pat. No. 5,535,522, entitled xe2x80x9cMethod and Apparatus for Determining the Alignment of Motor Vehicle Wheels,xe2x80x9d issued to Jackson, et al. on Jul. 16, 1996. The apparatus disclosed in these patents is sometimes called a xe2x80x9c3D alignerxe2x80x9d or an xe2x80x9caligner.xe2x80x9d
To align motor vehicle wheels, such 3D aligners use cameras that view targets affixed to the wheels. In one approach, one camera is used to view targets on one side of the vehicle, and another camera is used to view targets on the other side of the same vehicle. Unfortunately, in cases involving multiple wheels on one side of the vehicle that are distant apart, the field of view of the camera has to be fairly large to cover all of the wheels. This large field of view requires a larger target to subtend additional pixels to be viewed by the camera because a larger field of view with a smaller target renders inaccuracies in the alignment measurements. These larger targets take additional space, costs more, and, in many cases, are clumsy to handle. This is specifically true when there are multiple targets to handle. Further, the larger target requires that it be placed far away from the camera. In one arrangement, at least 8 feet is required from the camera to the head of the rack that supports the motor vehicle under alignment. Under these conditions, many smaller vehicle service shops cannot afford such a large space for the wheel aligner.
In addition, to accurately determine the position between the wheels on one side of the vehicle and the wheels on the other side of the vehicle, the aligner must know where one camera is positioned with respect to the other cameras. Therefore, the relative position of the two or more cameras must be measured and stored in a calibration process. According to one calibration method, a large target is positioned in the field of view of the cameras, typically along the centerline of the alignment rack, and away from the cameras. Information obtained from each camera is then used to determine the relative positions and orientations of the cameras. Since each camera indicates where the target is with respect to the camera itself, and since each camera is viewing the same target, the location and orientation of each camera with respect to the other camera can be calculated. Calculating the relative positions of the cameras is normally referred to as relative camera position (RCP) calibration.
Such calibration allows the results obtained from one side of the vehicle to be compared to the results obtained from the other side of the same vehicle. Thus, by mounting the two cameras rigidly with respect to each other and then performing an RCP calibration, the system can be used to locate the wheels on one side of the vehicle with respect to the other side of the vehicle from that point on. The RCP transfer function is used to convert one camera""s coordinate system into the other camera""s coordinate system so that a target viewed by one camera can be directly related to a target viewed by the other camera. One approach for performing an RCP is disclosed in U.S. Pat. No. 5,809,658, entitled xe2x80x9cMethod and Apparatus for Calibrating Cameras Used in the Alignment of Motor Vehicle Wheels,xe2x80x9d issued to Jackson, et al. on Sep. 22, 1998.
While RCP calibration is accurate, it requires special fixtures and a trained operator to perform. Thus, there is a need for an easier, simpler calibration process for an aligner. Further, even after calibration is performed, the aligner may be out of calibration. The aligner disclosed in the foregoing patents has cameras mounted on a boom that is designed to minimize loss of calibration. However, if the cameras are jarred or dismounted, or if the boom itself is bent, the aligner will be out of calibration. The aligner cannot detect loss of calibration itself. Normally, loss of calibration is not detected unless a calibration check is performed. A long time may elapse before the aligner""s out-of-calibration is realized.
In addition, the boom is large, expensive and present an obstacle to vehicles entering and leaving the alignment rack. xe2x80x9cDrive-throughxe2x80x9d alignment approaches may be used wherein a vehicle is driven forward into a service facility, aligned, and then driven forward to exit the service facility. This enables other motor vehicles to queue up behind the vehicle being serviced, improving the speed and efficiency of alignment services. In one approach of drive-through alignment that has a rigid boom, it is necessary to raise vehicle passes through. This can be time-consuming, costly, and clumsy. Automatic self-calibration has been disclosed in a co-pending patient application entitled xe2x80x9cSelf-Calibrating, Multi-Camera Machine Vision Measuring System,xe2x80x9d by Jackson et al., Ser. No. 09/576,442, filed on May 22, 2000. However, there are different approaches for an aligner to be calibrated.
Based on the foregoing, it is clearly desirable to provide a self-calibrating, multi-camera aligner that is improved over the aligner disclosed in the above-mentioned patents.
Techniques are disclosed for providing a system that has a plurality of devices in which the position of a device of the plurality of devices relative to another device of the plurality of devices is self-calibrated. In one embodiment, the system is a five-camera aligner for use in aligning motor vehicle wheels. In this embodiment, the aligner includes a first camera pod having two alignment cameras and a calibration camera and a second camera pod having another two alignment cameras and a calibration target. Because the aligner has four alignment cameras and a calibration camera, the aligner is often referred to as a five-camera aligner. For illustration purposes, the first camera pod is herein referred to as the left camera pod and the second camera pod is herein referred to as the right camera pod. In one embodiment, the left camera pod is placed to the left of the vehicle under alignment, and the right camera pod is placed to the right of the same vehicle. The two alignment cameras of the left camera pod focus at the two left wheels of the vehicle, while the two alignment cameras of the right camera pod focus at the two right wheels of the same vehicle. In addition, the calibration camera on the left pod focuses at the calibration target mounted in the right camera pod.
In one embodiment, the positions of the calibration camera and of the two alignment cameras in the left camera pod relative to one another is predetermined at the time the aligner is manufactured. Similarly, the positions of the calibration target and of the two alignment cameras in the right camera pod relative to one another is also predetermined at the time the aligner is manufactured. At the work site where the aligner is used for aligning wheels, the calibration camera is used to measure the position of the calibration camera relative to the calibration target. Because the position of the calibration camera relative to the calibration target is known, the position of the left camera pod relative to the right camera pod is known. In addition, because the position of the first alignment camera relative to the second alignment camera in the left camera pod is known, and the position of the first alignment camera relative to the second alignment camera in the right camera pod is known, the relative positions of the four alignments cameras are known. The aligner is thus said to have been calibrated and ready for use in aligning wheels.
In one aspect of the invention, the two camera pods are mounted on two respective frames, which, in turn, are mounted in a rack that supports the vehicle under alignment. In this aspect, the frames may be moved from one rack to another rack. Consequently, a service station may want to buy only one set of two camera pods for use in multiple racks. Further, the camera pods are moved (raised or lowered) with the rack.
In another aspect, the frames on which the camera pods are mounted are designed such that the frames are foldable to be hidden in the rack or to be arranged along the sides of the rack, to save space.
In another aspect, the camera pods are attached to a pair of towers, and, with appropriate mechanisms, the camera pods are also moveable with the rack.
In another aspect, the position of the left camera pod relative to the right camera pod is determined by using any camera to measure the position of the calibration camera relative to the calibration target. In this aspect, the (left) calibration camera may be substituted by a (left) calibration target. The position of the left calibration target in the left camera pod relative to the right calibration target in the right camera pod is thus determined by using a calibration camera that is external to both pods.
In another aspect, the left camera pod includes a left calibration camera and a left calibration target while the right camera pod includes a right calibration camera and a right calibration target. The left calibration camera is used to measure the position of the left calibration camera relative to the right calibration target, and the right calibration camera is used to measure the position of the right calibration camera relative to the left calibration target. Consequently, the calibration data of the relative positions of the left camera pod and the right camera pod that is provided by the left calibration camera and the right calibration camera should be substantially close. If the two sets of calibration data are not within an acceptable amount, then an alert alarm regarding the aligner""s calibration is raised.
In another aspect, a xe2x80x9cmiddlexe2x80x9d calibration target is used to verify calibration of the aligner that is provided by the position of the calibration camera relative to the calibration target. The calibration data provided by the calibration camera relative to the calibration target is herein referred as the first set of calibration data while the calibration data provided by using the middle calibration target is herein referred to as the second set of calibration data. In order to obtain the second set of calibration data, the middle calibration target is placed such that its position can be determined by both an alignment camera in the left camera pod and by an alignment camera in the right camera pod. Further, the right camera pod is placed such that the position of the right calibration target can be determined by the left calibration camera. From the position of the middle calibration target relative to the left camera, the position of the middle calibration target relative to the right camera, the position of the left camera pod relative to the right camera pod is determined, which provides the second set of calibration data. If the difference between this second set of calibration data and the first set of calibration data is beyond a predetermined amount, then the aligner has failed the calibration. In one embodiment, the left and right cameras are rotated so that their lenses face the middle calibration target that is placed between the two cameras, and the two camera pods are brought closer together.